1. Field of Invention
The present invention relates to a fan and, in particular, to a fan capable of increasing the airflow rate and volume, reducing the noise and increasing heat-dissipating efficiency.
2. Related Art
As a result of the advancement in process technologies, circuit boards of many electronic products are now made to carry electronic components (e.g., central processing units, memories, integrated circuits and the like) at an ever-higher density. Because electronic components generate heat during operation, denser electronic components increases the temperature of the whole electronic product, which may cause abnormal operation of the electronic products or even damage to the electronic components thereof due to intense heat.
In the prior art, a fan is disposed in an electronic product so that air flow produced by the rotating fan can force convection to cool the electronic components thereof.
FIG. 1 illustrates a conventional fan 1 including a fan wheel 11 and a fan cowl 12. The fan wheel 11 comprises a hub 110 and a plurality of blades 111. Each of the blades 111 has a blade width W1 and extends from a side wall of the hub 110.
The conventional fan 1 has a narrower blade width W1. In order to maintain certain efficiency in producing the air flow, a large number of blades must be provided. For instance, the conventional fan 1 depicted in FIG. 1 has seven blades 111. As a result, the pitch between the blades 111 is inevitably reduced. Moreover, to get a strengthened flow rate and improved heat dissipation efficiency, the rotation speed of the fan wheel 11 must be increased. However, when the fan 1 operates at a high rotation speed, there is too small of a pitch between the blades which will cause disturbance to the air flow fields produced by the blades to each other or even cause annoying noise.
According to the above description, the conventional fan 1 increases the rotational speed of the fan wheel 11 at the cost of exacerbated noise. On the other hand, if the rotational speed of the fan wheel 11 is not increased, the flow rate would be too small, which would be inadequate to deliver the desired heat dissipation efficiency for the dense electronic components.
Further, since the performance of the electronic product has been sufficiently enhanced, the heat-dissipating device or system becomes one of the indispensable components thereof. If the heat generated by the electronic product can not be dissipated properly, the performance of the electronic product may be lowered and, even more, the electronic product may be damaged. The heat-dissipating device is more important to the micro-electronic components (e.g. an integrated circuit, IC). This is because the surface of the IC continuously reduced due to the increasing integration and progressive package technology, which results in larger accumulated heat energy per unit area.
In order to enhance the heat-dissipating efficiency, it is necessary to increase the number of blades. However, in practice, it is impossible to unlimitedly increase the number of blades because the impeller will be burdened with very high pressure in high rotation speed and the space is limited, which may cause the break of the blades, generate noise or cause the airflow turbulence. In addition, in the high speed rotation, the structure of the motor and the bearing have very large load, which may decrease the lifetime of the mechanism. Besides, the side walls of the fan frame totally closed, and the periphery of the inlet of the conventional fan has an oblique design. That is the periphery is inclined downwardly from the outside to the inside. When the impeller is in stall speed, the heat-dissipating efficiency is decreased and the noise wave is highly increased.